Tornado Alley has the perfect geographical and meteorological conditions to spawn tornadoes, and the most powerful and destructive are common at this time of year.
How do tornadoes form?
Warm moist air is drawn up from the Caribbean. Warm but dry air comes across from California and the final ingredient is cold air plunging down from the Rockies.
These all collide over the centre of the huge landmass of the United States and Tornado Alley is where this happens.
The warm air rises rapidly over the cold air and with a little vertical wind shear (winds changing direction as you go up in height) the rotation begins.
A narrow spinning column or ‘vortex’ will quickly form a funnel cloud, and when this touches the ground a tornado is born.
These spiralling vortexes create the most violent storms in the world. The perfect conditions come together in April and May.
What happened on Monday?
A cold front sank south and collided with warmer air across the Midwest and the south central states where the air was unstable (meaning storms were ready to grow).
This caused huge amounts of energy to be released into the atmosphere, generating lots of thunderstorms.
Before long, funnel clouds and tornadoes started forming, the biggest of which crossed Moore in Oklahoma on Monday afternoon.
It measured F4 on the Fujita scale - a measure of a tornado's strength with F5 being the maximum.
Although it is nearly impossible to measure the exact speed of the wind in a tornado, it was estimated to be around 200mph.
The astonishing thing is that it was up to two miles in diameter at its widest. That is two miles of ferocious winds with tonnes of swirling debris.
As it passed, it picked up trees like toothpicks and tossed cars like they were toys, flattening everything in its path.
Is this unusual for this part of the world?
No, it is very common during the late spring and early summer as the heat starts to build and cold air is still lurking from winter.
This is not the first time Moore has seen such a damaging tornado.
In 1999, it saw an F5 tornado with winds reported up to 298mph - the strongest wind gust ever recorded on Earth. Truly remarkable.
That tornado was not as wide as Monday's and this, combined with the fact that the area is now more populated, has meant much more damage despite the winds being less powerful.
Are there more storms on the way?
The good news is they are clearing from Oklahoma, but the bad news is they are travelling along the same cold front.
This is moving east with more thunderstorms forecast and tornado warnings in force for the states of Kansas, Missouri and Illinois.
What is the difference between a tornado and a hurricane?
Hurricanes are huge weather systems, which develop over warm waters and track across the Atlantic. Their strength is measured on the Saffir-Simpson scale.
They can be hundreds of miles wide and can have tornadoes forming within them.
Tornadoes are narrow columns of cloud from a few metres to a few miles wide.